Set RHUB repo as ubuntu-24.04 for s390x

I see the `r-hub/repos` repo has: - [ubuntu-22.04-s390x/4.1](https://github.com/r-hub/repos/tree/main/ubuntu-22.04-s390x/4.1) - [ubuntu-24.04-s390x/4.3](https://github.com/r-hub/repos/tree/main/ubuntu-24.04-s390x/4.3) But when I try to run the s390x workflow, it is actually looking for https://raw.githubusercontent.com/r-hub/repos/main/ubuntu-22.04-s390x/4.3